The present invention relates generally to improvements in the automatic equalization of voice-band communication channels rendering them suitable for high-speed data communication use. More particularly, it pertains to new and improved binary equalization networks wherein the amplitude and phase characteristics of various voice-band communication channels are automatically compensated for to nullify the varying degrees of attenuation and phase distortion characteristic of each different voice-band communication channel.
One of the most critical problems confronting the developers of modem devices that facilitate the use of voice-band communication channels for the high-speed transmission of digital data has been that of coping with the amplitude and phase distortion characteristic that is unique to each voice-band communication channel in the telephone network. This problem becomes increasingly difficult as the data speed increases because as the data speed increases the signal-to-noise ratio on the channel becomes evermore critical.
The prior art has attempted to overcome this problem by a variety of means. One method utilizes a passive RLC network at the receiver end. This network is designed to have attenuation and phase characteristic equal to the difference between the desired characteristics and the characteristics existing on the communication channel. The drawback behind this approach is that it does not provide any flexibility. The compensation network is only effective on the channel for which is was designed.
In order to circumvent this lack of flexibility, a compensation network known as a transversal filter is utilized. In this type of arrangement, compensation for amplitude and phase distortion is accomplished by applying the received signal to a tapped delay line. The output signal is the result of the summation of signals from several of the taps. Each tap is provided with the means of effectively multiplying its contribution by any value between .+-.1. This filter arrangement can compensate for a variety of amplitude and phase characteristics. A drawback in this type of system is that it is inherently slow.
Another well known type of equalizer operates on the principle of predistorting the transmitted pulses in such a way that upon reception the pulses will have the desired shape as a result of the characteristics of the transmission channel utilized. Here again however, the correction factor is static. Each communication channel must utilize its own phase distortion network.
These equalization techniques for voice-band communication channels that carry optimized eight phase, two amplitude modified eight-phase or four-phase modulated signals have been unable to provide equalization that is both flexible and can operate at speeds in the 50 millisecond range. The inability to equalize very quickly precludes prior art equalization techniques from facilitating the use of modems in controlled carrier operation or in a polling environment.